Conventionally, in an image sensing apparatus such as a digital still camera, an electrical signal (image sensing data) of a photographed image, which is obtained by photoelectrically converting the photographed image in an image sensing element such as a CCD, undergoes predetermined processing to record as electric (magnetic) information the resultant image information (image data) of the photographed image on an external recording medium (e.g., memory card or hard disk). Completely unlike a conventional silver halide camera which photographs an object image by film exposure, a digital still camera enables reproduction of a photographed image as an electrical signal after photography by performing image processing for an electrical signal of the photographed image as positional information and recording the processed signal.
In recent years, in digital still cameras, megapixel processing has been realized for faithful reproduction of an image. The reproduction image quality of an image photographed by a digital still camera has approached that by a silver halide camera.
However, image data of a photographed image recorded/reproduced in a digital still camera requires a large amount of information (data). In particular, in still images for appreciative viewing or ones whose details matter, decimation of image data using human visual characteristics cannot be performed, and thus a large amount of data is necessary for a frame of still image.
For example, in megapixel-class digital still cameras, a photographed image to be recorded has a large amount of image data. If 12-bit data of a four million pixel photographed image is recorded, the data amount exceeds 6 Mbytes. Such a large amount of data is not suitable both for recording and saving on an external storage medium such as a memory card or hard disk and for transferring between devices via communication interfaces. Thus, when a large number of photographed images are recorded, their image data amount needs be decreased. Under the circumstances, in digital still cameras, the image data of photographed images to be recorded is generally compressed before recording, thereby decreasing the data amount of the photographed images.
Image compression schemes for compressing image data include lossless compression which enhances the retentivity (reproducibility) at the cost of the compression ratio and lossy compression which enhances the compression ratio at the cost of the retentivity (reproducibility). As a lossless compression scheme, e.g., a method of expanding differences between a sequence of image data into run-length information to encode the information using a table is commonly known. As a lossy compression scheme, a method of suppressing the high-frequency information of image data by orthogonal transform such as DCT (Discrete Cosine Transform) to encode the information using a table is known. Both image compression schemes are classified as conventional JPEG schemes. However, a JPEG scheme generally means the latter scheme, and the former is known as one for TIFF images.
Conventionally, to avoid the influence of block noise due to irreversible compression or the like, lossless compression has mainly been employed as an image compression scheme for digital still cameras because photographed images desirably have high quality. However, a rapid growth in number of pixels of a photographed image in recent years has aroused user's demands for selection with a preference for the image data size, and many digital still cameras using lossy compression schemes have appeared. Some users who give priority to improvement in image quality of photographed images select a digital still camera comprising an image compression means using a lossless compression scheme by which an unprocessed image faithful to a photographed image can be obtained as a reproduced image. Other users who give priority to the data capacity (i.e., give priority to reduction in amount of data in recording a photographed image) select a digital still camera comprising an image compression means using a lossy compression scheme.
Image compression means for lossless compression and lossy compression can both be provided in a digital still camera. However, in a conventional digital still camera, either one of lossless compression and lossy compression needs to be selected for an image file (image data) to be obtained by photography before actual photographing operation to continuously photograph without interruption. To this end, the device is arranged to compress by either one of the image compression means respectively for lossless compression and lossy compression in accordance with the selection, and an image file is obtained by the selected image compression scheme. That is, in a conventional digital still camera, only one of lossless- and lossy-compression image files can be obtained upon one photographing operation.
This is because lossless compression and lossy compression adopt completely different compression processing methods and they cannot synchronize with each other. Upon one compression operation, lossless compression and lossy compression cannot be simultaneously performed to generate respective image files.